The present invention relates to a format structure of data sectors formed on a magnetic disk, and more particularly to a technology for reducing an area used for a preamble of each data sector to increase the storage capacity of a magnetic disk drive.
Some magnetic disk drives adopt a data-surface servo system, which is also called a sector servo system, in which servo control is performed by writing servo data to a recording surface to which user data is written. In a format structure of a magnetic disk, which is adopted in the data-surface servo system, each data area is formed between servo sectors that are discretely located in each data track in the circumferential direction. A plurality of data sectors are written to each data area. The magnetic disk drive is usually configured as a direct access storage device that is capable of writing and reading data on a data sector basis. The data sectors are not written with a write clock that is synchronized with the rotational speed of the magnetic disk. Therefore, in order to realize a direct access storage device, every data sector includes a preamble used for synchronization with a read clock.
At the time of writing, a data sector which includes a preamble and user data is written at a specified position with a write clock having a specified frequency. At the time of reading, for each data sector to be read, user data is read with a read clock being synchronized with a signal obtained by reading a preamble of the data sector in question. In order to synchronize the read clock with a clock obtained by reading the preamble, a phase-locked loop circuit is required to read a preamble having a specified pattern length.
The preamble includes a redundant part so as to cope with the jitter occurring in, for example, the rotational speed of the magnetic disk at the time of reading, and the timing of a read gate. Because the preamble is written to a data area, the preamble narrows a write area of user data. Therefore, it is desirable that the pattern length of the preamble be made as short as possible. Patent document 1 (Japanese Patent Laid-open No. 2004-280898) discloses an information writing and reading device in which a preamble is shortened to extend a write area. As shown in FIGS. 1(b) and 1(c) in patent document 1, a preamble of each data sector is divided into first, second, and third preambles.
Patent document 2 (Japanese Patent Laid-open No. 2005-100529) discloses a technology for reducing a synchronization area of a split sector. As shown in FIG. 3 in patent document 2, a synchronization area of a first split sector is divided into a phase synchronization area and a phase frequency synchronization area. However, a synchronization area of a second split sector forms only a phase synchronization area. A synchronizing frequency at the time of reading the first split sector is stored in a synchronizing frequency storage unit shown in FIG. 1 in patent document 2. When the second split sector is read, this stored synchronizing frequency and a pattern of a synchronization area are used.